1. Field of the Invention
This invention relates to a resin composition for making cores or molds by bonding foundry sand. 2. Description of the Prior Art
Phenolic resins are now in widespread use as sand binders in the production of foundry molds. These resins, however, pose a problem of environmental pollution because when heated during coating of sand, molding or pouring, they give off toxic or malodorous gases. When a phenolic resin-coated sand is used in making a core for casting an aluminum alloy, the disintegrability of the core is very poor. Specifically, the heat (650.degree.-750.degree. C.) of the molten metal after casting causes hardening of the phenolic resin at a thick portion of the inside of the core so that this portion attains an increased strength. On the other hand, a thin portion of the core is placed in an oxygen-free condition when surrounded by the molten metal. Hence, the benzene ring of the phenolic ring mainly undergoes carbonization to cause bonding between said particles or between the sand particles and the aluminum alloy casting, and therefore, the strength of the thick portion scarcely decreases. In the production of an aluminum alloy casting using a core of a complicated profile made by phenolic resin-coated sand, therefore, it is usual to facilitate disintegration of molds and cores by a shake-out machine by preliminarily baking the mold assemblies containing castings for a period of as long as 4 to 10 hours at 400.degree. to 500.degree. C. It is thus not always desirable to use a phenolic resin as a resin component of a resin composition for bonding of foundry sand.